Passive compliance has become an increasingly important aspect of robotic and rehabilitation systems. Classically, robots have relied on stiff appendages and precise position control of joints to facilitate high-speed trajectory tracking. However, many applications benefit from an alternative approach that relies on inherent compliance to improve performance.
Biologically-inspired robots have long included passive compliance as a key design element. Running, hopping, climbing and perching robots have been designed where appropriate selection of joint and appendage impedance leads to reduced shock forces, increased robustness, and increased efficiency via energy storage and release. Such strategies are inspired in part by animals' ability to vary joint impedance via co-contraction of antagonistic muscles. Active impedance control with stiff actuators is possible, but is limited by bandwidth, weight, and power consumption.
Human-safe robot operation also shares similar requirements. In industrial robot manipulators, passive compliance helps prevent humans from experiencing high forces during accidental contact. In rehabilitation devices, impedance matching with the patient is necessary for many tasks. Here, passive compliance promotes “fail-safe” operation when compared to active impedance control.
Passive compliance can be achieved through devices such as the series elastic actuator (SEA), combining a passive spring and a stiff motor. Advantages include low weight and few moving parts. However, their ability to vary this compliance is still limited to active control of the serial motor. To achieve variable passive compliance, several different techniques can be used, broadly categorized into antagonistic systems and structure-controlled systems.
Antagonistic systems rely on manipulation of nonlinear springs to change their equilibrium position. An example such as AMASC can independently control joint position and stiffness. These systems, while similar to the biological strategy of muscle co-contraction, have disadvantages in compact robotic devices due to their motor size requirements, power usage, mechanical complexity, and weight.
Structure-controlled systems exploit a change in passive spring geometry or coupling. Varying the effective length of a spring or the moment of inertia of a beam are common methods to achieve this. While they use less power to change stiffness and are much simpler mechanically, these systems still have moving parts and often a heavy or bulky actuator, precluding their use in applications with tight mass or volume constraints.
Electroactive polymers have been described as “artificial muscles” due to several muscle-like properties, such as inherent passive compliance and damping, low weight, flexible geometry, and silent operation. They have been examined most often as a prime mover actuator, with very high strains and forces possible using careful design and multiple film layers. However, their disadvantages include high voltage requirements, low bandwidth due to hysteretic losses, and actuator failure due to manufacturing defects, mechanical film overstrain and tearing, and dielectric breakdown and shorting.
What is needed is a variable stiffness spring that is light-weight, flexible and highly responsive.